1. Field of the Invention
The present invention relates generally to asphalt compositions and more particularly to a roofing shingle utilizing a specific asphalt composition.
2. Description of Prior Art
Most roofing shingles made today are constructed of a base sheet, usually either rag felt or, more recently, glass fiber mat, saturated with a bituminous substance, usually asphalt, including a large amount of filler material such as limestone dust. Roofing shingles of this general type are made in various different sizes and shapes and utilize asphalt compositions and base sheets which differ greatly from one another. In this regard, the various competitors making up the roofing industry are constantly striving to decrease the manufacturing costs of their shingles while, at the same time, striving to improve shingle strength, weatherability and overall general quality. One feature which, in recent years, has been somewhat overlooked is shingle fire-resistance. For the most part, this is because many manufacturers already have a "fire-resistant" asphalt shingle having a "Class A" rating from Underwriters Laboratories, the highest UL rating for fire resistance that can be presently obtained.
In order to keep the cost of asphalt shingles at a competitively low level, most manufacturers use an inexpensive low-viscosity asphalt and relatively inexpensive filler material such as limestone in manufacturing their shingles. To improve upon shingle fire-resistance, many manufacturers have found it necessary to incorporate some type of fire-resistant improving additive to the asphalt-filler combination. One typical additive is asbestos and another is ferric chloride (FeCl.sub.3). A major problem in utilizing asbestos is that it is at least thought to be a hazardous material and thereby requires expensive equipment to maintain acceptable air quality standards during manufacture.
A major problem with FeCl.sub.3 is that it is a corrosive material and FeCl.sub.3 asphalts tend to form troublesome coats and skin during shingle manufacture. However, without any additive, that is, utilizing an asphalt composition including only asphalt of the inexpensive and low-viscosity type typically used and filler material such as limestone, it is difficult to provide a highly fire-resistant shingle without, for example, utilizing an expensive base sheet. The major reason for this resides in the viscosity or flow characteristics of the asphalt used.
More specifically, shingled roofs, unlike built-up roofing decks, lie at a slope with the horizontal. When the shingles are subjected to a fire, the asphalt melts and reaches high temperatures rather rapidly. At these temperatures, without any additive other than the conventional filler, the asphalt has a low viscosity and flows quite rapidly down the roof slope and away from the source of fire. In so doing, it carries much of the filler and base material with it. This, in turn, leaves any hot spots on the support deck exposed to the air and particularly any breeze or wind. By utilizing an additive such as asbestos or FeCl.sub.3, the asphalt is sufficiently viscous such that a protective coating or crust is formed over the hot spot acting as a shield against the outside air. This protective coating or crust results from the burned remains of at least a portion of the combustible ingredients comprising part of the asphalt composition.